Gas sensors for sensing oxygen are used in a wide variety of applications where the amount of oxygen must be measured. Examples of uses for such oxygen sensors include use in fuel-air mixtures and exhaust systems in combustion engines, gas burning appliances, for use in measurement of oxygen levels in blood and in regulation of anesthesia, and environmental applications. One known technique for measuring oxygen is a ceramic based sensor. Ceramic base sensors are commonly used in the automobile industry for combustion exhaust monitoring. Examples of ceramic based oxygen sensors include U.S. Pat. Nos. 4,547,281 to Wang et al, or 4,950,380 to Kurosawa et al. These types of ceramic sensors are disadvantageous in that they require operating temperatures approaching 400.degree. C. to be effective, which may make it inappropriate for certain applications. It would be desirable to have an accurate oxygen sensor having both good stability characteristics and be operable at temperatures significantly below 400.degree. C.
Another known technique for measuring oxygen in an external environment is a galvanic cell type oxygen sensor, where typically a liquid electrolyte, often an acid in an aqueous solution, is positioned between positive and negative electrodes (a cathode and an anode) and a current is passed between the electrodes. Oxygen is presented at the cathode. An oxidation reaction occurs at the anode and a reduction reaction occurs at the cathode. The potential difference between the electrodes is proportional to the concentration of oxygen sensed at the cathode. Such sensors are capable of measuring oxygen concentrations near room temperature, commonly in medical and environmentally applications. Examples of such galvanic cell or liquid electrolyte oxygen sensors include, for example, U.S. Pat. Nos. 4,495,051 to Fujita et al, 4,775,456 to Shah et al, 4,988,428 to Matthiessen et al, and 5,284,566 to Cuomo et al.
Although such liquid electrolyte oxygen sensors work at ambient temperatures, such sensors have numerous problems. The chemical reaction of the liquid electrolyte tends to run fairly quickly, limiting the total operational lifespan of such oxygen sensors. Moreover, the rate of reaction, which affects potential difference between the electrodes, is a function of the concentration of the liquid electrolyte, and the concentration of electrolyte changes as the reaction runs. Further, the concentration of liquid electrolyte changes as it dries out over its service life. This means that such oxygen sensors need to be regularly recalibrated, often on a daily basis, to account for the change in concentration of the electrolyte. Compensation for such problems drives up the cost of the sensor and reduces their effectiveness. It would be highly desirable to provide an oxygen sensor which would last for an extended period of time and not require continuous recalibration.
In view of the foregoing, it is an object of the present invention to provide a gas sensor for sensing oxygen having a long operational lifespan which is highly reliable in operation and does not need to be recalibrated after initial setting. It is an additional object of the present invention to provide an oxygen sensor for use in an oxygen detector having fast response times. It is an additional object of the present invention to provide an oxygen sensor which is of low cost, compact size and is easy to manufacture.